When the user connects to my application, executes one stored procedure to register the user login. This stored procedure update one table that force the load of most of the metadata in memory.
The difference is that while Firebird 2.1.5 uses 57100 Kb, Fb 2.5.1 uses 84100 Kb, that is a 47% more memory than 2.1. once executed the procedure and starting to work with the application, the memory consumption increases in parallel. That implies that for 200 users the less memory needed for 2.5 is near 16 Gb, while for 2.1 is near 11 Gb that is a big difference.
That implies that classic server will use per connection so much memory. I don't know if is a bug or not, but the difference is so much. If classic server is for complex enviroments with high load, then
Steps to reproduce the issue: Connect to the database and execute procedure usuario_entrada( 0, '', 'STATION01', '' )